The present invention relates to a method for making polycarbonate by phosgenating a bisphenol in the presence of a phase transfer catalyst (PTC). More particularly, the present invention relates to the employment of a PTC, such as a tetraalkylammonium or tetraalkyl phosphonium halide, as a condensation catalyst for the polymerization of a bisphenol under interfacial reaction conditions in combination with a tertiary organic amine.
As shown by Freitag et al, The Encyclopedia of Polymer Science and Engineering, John Wiley & sons, New York (1988), Second Edition, polycarbonates have been made by phosgenating bisphenols under interfacial reaction conditions using a tertiary organic amine, such as triethylamine, as a condensation catalyst. Although tertiary organic amines have been found to be effective as bisphenol condensation catalysts, experience has shown that use of tertiary organic amines results in excess phosgene usage. Efforts have been made therefore to develop procedures to minimize excessive phosgene losses. For example, a reactor with an overhead condenser can be used to trap phosgene, as shown by Silva, U.S. Pat. No. 4,701,544, or the rate of heat generated during the course of bisphenol condensation can be monitored, as shown by Brunelle et al, U.S. Pat. No. 4,814,420.
Although tertiary organic amines have been found to be highly effective as condensation catalysts for building polycarbonate molecular weight during phosgenation, experience has shown that tertiary organic amines often lead to excessive phosgene usage as a result of phosgene hydrolysis, and/or chloroformate hydrolysis prior to the termination of polymerization. For example, in a phosgene hydrolysis rate study, it was found that at a triethylamine concentration of 6.64.times.10.sup.-3 M, triethylamine catalyst effected phosgene hydrolysis at a relative rate of greater than 200 compared to a reference value of 1 without any catalyst. With respect to the rate of chloroformate hydrolysis, where a relative value of less that 0.01 has been found for a catalyst-free system, triethylamine shows a value of greater than 100.
In Campbell et al, U.S. Pat. No. 4,471,105, an interfacial polycarbonate poly condensation process is shown for making polycarbonates from sterically hindered bisphenol precursors utilizing a quaternary phosphonium amino halide catalyst. Additional investigations with respect to phase transfer catalyst usage demonstrate the syntheses of copolycarbonates (carbonate-thiocarbonate)s from bisphenols and a mixture of phosgene and thiophosgene are shown by Tagle et al, European Polymer Journal 1987, 23(2), 109-112, and European Polymer Journal 1989,26(6), 549-551.
Various condensation catalysts are constantly being evaluated to determine their effectiveness for polymerizing various types of polymers. Considerable effort is being maintained to determine the effectiveness of such condensation catalysts as replacement condensation catalysts for tertiary organic amines in conventional polycarbonate synthesis.